Various freeze processes and apparatus have been developed to produce potable water from seawater or brackish water; to concentrate fruit juices such as orange juice, grape juice and apple juice, vegetable juices such as tomato juice, and coffee; and to separate dissolved or suspended solids from a liquid carrier. See, for example, Engdahl et al U.S. Pat. Nos. 4,286,436; Ashley 3,501,924; Ganiaris 3,620,034; Johnson 3,664,145 and Ogman 4,091,635.
After an ice slurry has been produced, it is usually desirable to separate the ice from the liquid and to wash it with pure water. When potable water is being produced from seawater or brackish water, the washing is necessary to remove salt water solution residues from the ice surfaces. In the concentration of various products, such as a fruit juice, the ice is washed to recover any juice residue clinging to the ice crystals.
Apparatus for separating an ice slurry and washing the ice for the described purposes has been known in the art for a considerable amount of time. One type of such apparatus includes a vessel having a bottom and cylindrical wall with an open top. Vertical tubes with perforations in their upper portions are positioned in the vessel. An ice slurry fed to the vessel separates into a pack of ice floating on concentrated liquid. The concentrated liquid flows through the tube perforations and is collected if the liquid is a desirable product, such as a juice, drained off to waste or recycled in the process. Pure water is sprayed onto the top of the ice pack to wash it clean. As the washed ice pack rises above the vessel top edge it is scraped off and collected. Melting the washed ice yields potable water. If potable water is not the desired product, such as in a juice concentration process, the ice can be melted for recovery of refrigeration or it can be discarded. An apparatus of the described type and its operation are disclosed in U.S. Pat. Nos. 4,341,085 and 4,517,806.
When a slurry is fed to the ice separating and washing vessel, a pressure force is created in the vessel which drives the crystal pack upwardly due to upward flow of the concentrated liquid through the forming ice crystal pack. As the total liquid flow or ice crystal fraction changes, the concentrated liquid flow is affected. At times this creates a condition where the driving force increases while crystal pack formation is slowed. As a result, the crystal pack formation zone moves, causing variations in crystal pack density, porosity and draining characteristics.
In order to obtain ice pack washing uniformity, it is desirable for the ice pack to have a height within a predetermined rather narrow range. This requires that the location of the ice pack bottom surface be known so that the rate at which ice slurry fed to the vessel can be regulated so as to approximately balance the amount of ice fed to the wash vessel with the amount of washed ice removed therefrom and also to coordinate removal of excess concentrated liquid from the vessel.